Attention has been drawn to a hydrogen-oxygen fuel cell as a power generating system which presents substantially no adverse effects on the global environment because in principle, its reaction product is water only. Polymer electrolyte fuel cells were once mounted on spaceships in the Gemini project and the Biosatellite project, but their power densities at the time were low. Later, more efficient alkaline fuel cells were developed and have dominated the fuel cell applications in space including space shuttles in current use.
Meanwhile, with the recent technological progress, attention has been drawn to polymer fuel cells again for the following two reasons: (1) Highly ion-conductive membranes have been developed as polymer electrolytes and (2) it has been made possible to impart extremely high activity to the catalysts for use in gas diffusion electrodes by the use of carbon as the support and an ion exchange resin coating.
Accordingly, extensive studies have been conducted on a process for producing an electrode/polymer electrolyte membrane assembly (hereinafter referred to simply as an assembly) for polymer electrolyte fuel cells.
The polymer electrolyte fuel cell which is presently being studied, has a low operation temperature of from 50 to 120° C., and therefore is defective in that exhaust heat can hardly be utilized effectively for e.g. an auxiliary power for electrolyte fuel cells. For the purpose of compensating such a defect, polymer electrolyte fuel is cells are required to have a particularly high output power density. Further, as an object for the practical use, it is required to develop an assembly which can give a high energy efficiency and a high output power density even under an operational condition of high utilization of fuel and air.
Under such operational conditions of a low operation temperature and high utilization of gas, especially in a cathode where water is formed by the cell reaction, clogging of an electrode porous body due to condensation of water vapor (flooding) is likely to occur. Accordingly, in order to obtain long-term stable properties, it is necessary to secure water repellency of the electrode so as not to cause flooding. This is particularly important for a polymer electrolyte fuel cell which gives a high output power density at a low temperature.
In order to secure the water repellency of the electrode, it is effective to reduce the ion exchange capacity of an ion exchange resin which coats a catalyst in the electrode, namely, to use an ion exchange resin with a low content of ion exchange groups. However, in such a case, the water content of the ion exchange resin tends to be low, whereby the electroconductivity decreases, and the cell performance decreases. Further, the gas permeability of the ion exchange resin decreases, whereby the supply of the gas to be supplied to the catalyst surface via the ion exchange resin coating will be slow. Therefore, the gas concentration in the reaction site decreases and voltage loss increases. Namely, the concentration overvoltage increases, and the output power decreases.
Accordingly, it has been attempted to use a resin having a high ion exchange capacity as an ion exchange resin which coats a catalyst, and in addition, to incorporate a fluororesin, such as a polytetrafluoroethylene (hereinafter referred to as PTFE), a tetrafluoroethylene (hereinafter referred to as TFE)/hexafluoropropylene copolymer or a TFE/perfluoro(alkyl vinyl ether) copolymer, or the like, as a water repellent agent, in the electrode, especially in the cathode, thereby to suppress flooding (see, for example JP-A-5-36418). In this specification, “an A/B copolymer” means a copolymer comprising repeating units based on A and repeating units based on B.
However, if the amount of the above water repellent agent in the electrode is increased so as to have sufficient water repellency, the electrical resistance of the electrode increases, because the above water repellent agent is an insulator. Further, there is a problem that the gas permeability decreases due to an increase of the thickness of the electrode, and the output power rather decreases. In order to compensate the decrease of electroconductivity of the electrode, it is necessary to increase the electroconductivity of e.g. a carbon material as a carrier for the catalyst or the ionic conductivity of the ion exchange resin which coats the catalyst. However, it is difficult to obtain an electrode which satisfies both sufficient electroconductivity and sufficient water repellency, and thus, it was not easy to obtain a polymer electrolyte fuel cell with a high output power and long-term stability.
Further, a method of mixing fluorinated pitch (see, for example, JP-A-7-211324) and a method of treating a catalyst carrier by fluorination (see, for example, JP-A-7-192738) have been also proposed, but there is a problem that the surface of the catalyst can not uniformly be coated with an ion exchange resin. Still further, a method of letting the water repellency have a gradient in the thickness direction of the electrode (see, for example, JP-A-5-251086 and JP-A-7-134993) has been proposed, but the production process tends to be cumbersome.
In order to increase the output power of the fuel cell, it is necessary that the ion exchange resin in the electrode has high gas permeability and high electroconductivity, and such an ion exchange resin preferably has a high concentration of exchange groups and a high water content. However, if such an ion exchange resin having a high concentration of exchange groups is used, flooding tends to occur, and the output power tends to decrease during long-term use though the initial output power becomes high by virtue of the high permeability of the fuel gas and electroconductivity.
In order to solve such problems, the present inventors proposed a perfluoropolymer having alicyclic structures in its main chain (JP-A-2002-260705).
Although improvement was made as compared with a linear perfluoropolymer having sulfonic acid groups usually used for a polymer electrolyte fuel cell, such a perfluoropolymer having alicyclic structures in its main chain was not sufficient for durability or the like if it was exposed to more severe conditions.